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Logic puzzle.... how many bus bars?
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paulca:
Having had chat GPT fail at this spectacularly.  I thought test you guys out.

to create a battery with 8S2P cell configuration, how many bus bars is the minimum required?

As this is a lithium pack, the cells need to be in parallel first, then the parallel groups put in series.  This is the part I could not get ChatGPT to understand.

The obvious answer is, you need 2 busbars for each cell pair to put them into parallel  so if you have 8 pairs you need 16 bus bars.  Then additionally to put the 8 groups into series you need another 7 bus bars.

Part of me however is saying ,"There has to be a mathematical trick to this, if you lay them up a specific way you might save ordering a few bus bars."

What say ye?
Nusa:
In fact it's quite common to lay out battery packs to share bus bars. 1S = 2 bus bars. 2S = 3 bus bars. nS = n+1 bus bars. 8S = 9 bus bars. The parallel aspect doesn't change things, it just makes the bus bars bigger.
paulca:

--- Quote from: Nusa on July 04, 2023, 02:52:21 pm ---The parallel aspect doesn't change things, it just makes the bus bars bigger.

--- End quote ---

True.  What if I added that you already have single link bus bars (the majority of) and the cost of buying/fabbing common bars shared between many cells (like 7 arge ones) is excessive (£150+) for copper and ....  you'd have to have them fabbed for you if you want them plated.

i can buy these bus bars, but either a 2-4 week wait from china or £5 a pair locally.
paulca:
Alternative to buying more bus bars include:

Buy a second BMS and put the packs in parallel.  cost ~£100 + shipping wait time from china.
Fabricate the missing "bus bars" from 6AWG wire and crimp lugs.
Nusa:
Sounds like where you went wrong is failing to adequately describe the physical aspects of the batteries you had in mind. Still haven't, really, but crimp lugs suggest you're talking about large cells with screw terminals.

The problem as originally described makes one think of 18650-like packs, which typically have cells spot-welded to bus bar plates or the equivalent made up of nickel straps. The spot welder for this kind of task is in the $50 range these days. I've put together a fair number of tool battery packs. (The very first one I ever did was soldered, and it works. But that's not recommended, and I knew it.)
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